Gas Exchange System in Bony Fish

Introduction

• Bony fish are a large group of fish with a skeleton made of bone, including species like tuna, salmon, cod, and trout.
• Bony fish have a high oxygen requirement due to their size and activity level.
• They have evolved a specialized gas exchange system to maximize oxygen extraction from water.

Structure of the Gas Exchange System

• Operculum: A flap of tissue on either side behind the head, covering the gills.
• Opercular Cavity: Contains the gills.
• Gills:
  • Oxygen-rich water enters through the mouth, passes over the gills where oxygen diffuses into the blood, and carbon dioxide diffuses into the water.
  • Water exits through the opercular opening.

Detailed Structure of Gills

• Gill Arches: Bony structures from which gill filaments extend.
• Gill Filaments: Many pairs extend from each gill arch.
• Gill Lamellae/Plates: Cover the gill filaments where actual gas exchange occurs.
  • Adaptations for gas exchange:
    • Massive surface area for diffusion.
    • Very short diffusion distance through lamellae walls.
    • Extensive network of blood capillaries to carry away oxygen and maintain concentration gradient.

Counter Current Exchange System

• Flow Direction: Blood flows in the opposite direction to water flow.
• Advantages:
  • Maintains a steep concentration gradient for oxygen.
  • Prevents equilibrium, allowing up to 80% of oxygen to diffuse into the bloodstream.
• Parallel Flow Issue: In parallel flow systems, equilibrium is reached quickly, limiting oxygen diffusion to 50%.

Constant Water Flow Mechanism

• Non-Bony Fish: Water flow occurs due to swimming.
• Bony Fish:
  • Have a mechanism to maintain water flow without swimming.
  • Process:
    • Open mouth: Water enters buccal cavity.
    • Buccal cavity floor drops, increasing volume.
    • Shut operculum, increase opercular cavity volume.
    • Pressure in opercular cavity falls.
    • Buccal cavity floor lifts, increasing water pressure and flow over gills.
    • Close mouth, open operculum, squeeze opercular cavity, increase water pressure to expel it through the operculum.

Conclusion

• Bony fish have complex adaptations for efficient gas exchange, crucial for their survival in water with low oxygen concentration.